Ophthalmic lenses to correct human vision have been in use for centuries. Nonetheless, new developments in materials and in optical designs continue to offer more options and various improvements to lens wearers.
As one example, consider the history of developments to address age-related, reduced focal accommodation—the common phenomenon of “needing reading glasses” as one gets older. The eye's lens is held within a sophisticated framework of muscles and fibers, and is pliable enough to be reshaped by the contraction and relaxation of the muscles and fibers. Thus, the action of the muscles and fibers change the shape and therefore the focal length of the eye's lens. The “rest state” is for distant vision, with the muscles relaxed and the lens in a less curved configuration for a longer focal length. For near vision, the muscles tighten and the lens becomes more rounded to bring into focus near objects. When one looks between a distant scene and a near object, the eye automatically tries to adjust muscles in the eye to refocus the eye's lens. However, as one ages, the lens begins to harden, and does not respond as readily to the muscular changes. This reduced accommodation, formally known as presbyopia, prompts the need for some vision correction to assist the eyes.
A simple approach for those with no previous vision correction is to use single vision glasses that provide more power for near-viewing tasks. However, if one then looks through the lenses toward a distant object, the view will be blurred because the lenses are causing one's natural vision to become myopic. A similar effect is seen when looking through a magnifying glass at distant objects. At that point, one quickly removes the glasses which can lead to the problem of misplacing them. As an alternative, one thinks of Benjamin Franklin and his bifocal lenses. Such lenses have a primary surface curvature for distance correction (if necessary) plus an added segment that provides more power for near-viewing tasks. To achieve the higher power, the added segment has a steeper lens curvature, and therefore this segment protrudes from the primary lens surface. Many people object to bifocal lenses because of this visible line and ledge on the spectacles.
Initial efforts to blend the region between the distance and near viewing zones (blended bifocals) raised greater awareness of a wearer's ability or inability to tolerate off-power areas on the lens. Many eyeglass wearers' can tolerate low power errors (typically less than about 0.5 diopters), but others may be extremely sensitive to power variation. This can be particularly problematic for individuals that had “perfect vision” and now, for the first time, need vision correction for near viewing. Any change from perfection seems extreme, even if the actual power variation measurement is very small. Similarly, work on blended designs revealed wearer's sensitivity to the physical locations of off-power regions on the lens, and to the physical size of the off-power regions. In the blended design, there is a narrow area, typically only a few mm wide, between any distance correction and the higher-powered, near viewing zone of the lens. Yet in this narrow area, the power must change at least as much as the difference between the distance and near viewing powers. Since the increased optical power or “add power” for the near-viewing zone is typically in the range of >0.5 diopters to about 4 diopters, this means that most people will note blurriness as the eye crosses over the blended region. This can be quite annoying to users, even if the blended area is not visible to others looking at the lens wearer. This led to further developments designed to minimize the area of such off-power regions, make the power changes more gradual so they could be tolerated, or push them toward locations on the lens that are less often used (such as the periphery).
For example, patents such as U.S. Pat. Nos. 2,109,474 and 2,475,275 have described lenses with one surface having gradually increasing power (changing radius of curvature) to give the user a range of focal lengths across this lens. These lenses may include spherical regions of constant power on the surface with the gradual power increase, like the bifocals and blended bifocals mentioned above. The other side of the lens is described as being ground to prescription, which typically means the distance-viewing correction, and at the time these patents were granted, the grinding expertise would be effectively limited to spherical and cylindrical shaping of the other surface. This means much of the lens could suffer from the same limitations as the blended bifocals, namely blurred vision due to the continuous increase of power.
The limitations and difficulties encountered with these previous techniques re-directed developments of lens design over the last several decades to other approaches. As evidenced by patents such as U.S. Pat. Nos. 3,711,191, 4,253,747, 4,472,036, and 6,019,470, one could make a lens that includes one area of stabilized power for distance viewing, another area of stabilized power for near viewing, and a typically narrow region between these two zones where the power is continually and gradually changing from one of these values toward the other. Lenses with these three regions are commonly referred to as progressive lenses.
It is common in a progressive design is to have a distance viewing area near the top of an eyeglass lens. As an example, assume that the wearer needs a moderate correction for farsightedness of 2 diopters. Then in the distance viewing area, the corrective power of the wearer's lens will be 2 diopters. Now, for example, assume that the wearer is an emerging presbyope, and needs a slight reading power assistance of 1 diopter, sometimes referred to as an add power of 1 diopter. Therefore, in the second, near viewing area of the lens, the stabilized power will be 3 diopters (2 diopters for overall vision correction, plus 1 diopter for near-vision additional correction). The near viewing area is typically positioned near the bottom of the lens, and often slightly toward the nose; this is consistent with a wearer looking downward toward a hook or hand work, and the slightly inward positioning accommodates the binocular tracking of the eyes for a near vision area. Typically, one tries to design the distance-viewing area and near-viewing area to be as large as possible, so the user has “plateaus” of nearly constant, stabilized power for their distance and near eyesight corrections. In particular, the distance viewing area should be large because of the width of viewing angle one may use. The near-viewing area may be smaller, but still must accommodate at least the width of the pupil for clearest reading vision, and preferably subtends a small angle for some eye rotation while reading; thus it is common to make the near-viewing area of stabilized add power at least a few millimeters wide. Between these upper and lower areas, the optical power must change rapidly to the higher, near-viewing value. In this example, that is a change from 2 diopters to 3 diopters. This progressive region or corridor will be characterized by an inflection point and is typically kept relatively narrow and short, because it is neither the desired distance nor near power, and because of physical necessity, as will be explained below.
There is a further complication to achieving such a progressive power increase. The physical surfaces of the lens must be reshaped and more sharply curved in order to create a higher-powered region. In the process of reshaping part of a surface to higher power, other areas with off-power values (and optical astigmatism) will be created. A rough analogy can be made to moving sand in a sandbox, without the option of removing or adding sand to the box. Therefore, in order to make a hill (analogous to an area of higher power), sand must be piled up in one area, but scooped out in other areas. If one wants to keep more of the sandbox at the original level (analogous to an original distance-viewing power), then one must widen the area scooped out to lessen the amount of difference in its height from the rest of the sandbox. However, this means that a larger area has SOME variation from the prescribed distance power, and as indicated previously, some individuals may be quite sensitive to such power variations. Alternatively, one can scoop deeply in a smaller area, but that will obviously create a zone of more extreme off power (greater difference from the original sandbox level). These problems become more severe as the difference between the two optical powers increases (a higher “hill”). These are practical, mechanical and physical limitations associated with lens designs that incorporate changes in optical power.
As an alternative, U.S. Pat. No. 4,950,057 describes the combination of stepped Fresnel optics with refractive lens surfaces to create different optical power regions. This is a distinctly different approach than using only the refractive capabilities of lens materials, and can encounter limitations due to the Fresnel discontinuous multi-step patterns. For example, there can be increased light scattering off the Fresnel steps, which can be annoying for the wearer and unaesthetic in appearance. There may also be distortion or lack of optical clarity in crossing over the multiple stepped structures.
As described in some of the previously mentioned patents and as known to those of ordinary skill in the art, progressive designs can be incorporated on either the outer lens surface (the surface farthest away from the wearer, or the “front” of the lens) or the inner surface (nearest the eye, or the “back” surface) of an eyeglass lens. This is often accomplished via “progressive semi-finished lens blanks” that incorporate on one surface a relatively large, effective stabilized distance-viewing area, another near-viewing area with a known, stabilized power that is greater than the distance power, and a relatively narrow, short corridor running between these two zones (the intermediate section) that is characterized by a progressive increase in optical power and an inflection point. The individual's wearer prescription is then “finished” by cutting and smoothing the other, opposite surface of the semi-finished blank to the specific optical power requirements of the user. With progressive semi-finished lens blanks, this typically means finishing the other lens surface for the distance-viewing correction and using the progressive surface to supply all the near-viewing correction.
Alternatively, both surfaces may incorporate progressive designs, as described, for example, in U.S. Pat. Nos. 4,946,270, 6,935,744 and 7,399,080. Another alternative but related approach is described in patents such as U.S. Pat. Nos. 6,139,148 and 7,159,983, in which one surface is a progressive design and the other surface is a “regressive” surface, that is, a surface where power decreases between the distance-viewing area and the near-viewing area. These regressive surfaces may be placed on either the inner or the outer surface of the lens.
Placing the add power on the inner surface of the lens, or sharing the prescription power between both surfaces allows more freedom in optical design and may have advantages for cosmetic appearances. These options have been further assisted in the marketplace by the growth of digital surfacing equipment, based on CNC machinery, which has the potential for more complicated and controlled shaping of one or both optical surfaces.
Given the continued need for age-related vision corrections, and the fashion-consciousness of many people, it is not surprising that optical performance, physical comfort and cosmetic appearance are all quite important. These factors have played an important role in the re-emergence of inner surface progressives, which may appear less obvious or bulky to someone looking at the person wearing the lens, because the add portion does not bulge forward from the outer surface. Nonetheless, such lenses can be problematic for the wearer, because of the need to accommodate the increased add power toward the eye of the wearer. This means the back surface of the lens will be less curved (less concave) than for single vision prescription, or than for a lens having progressive power on the front (outer) surface. For a high plus prescription, the back or inner surface of even a single vision lens is much flatter than for a high minus powered lens. If one then adds the near-vision power to the inside bottom section of a plus lens (as in a standard back-side progressive design), the inner surface of the lens becomes even flatter, and may contact the cheeks or eyelashes of the user. One way to avoid or minimize this problem is to use a lens with a steeper front curvature, so there is more space to create the add power on the inner surface of the lens. However, the steeper curve will require more lens material to create the same add power as a combination of flatter curves, and the result can be an overall heavier or thicker lens. In addition, using a more curved lens negates the advantage that was being sought: to reduce the bulbous appearance of the front-side progressive.
The same difficulties apply with high minus prescriptions and an add power on the inner surface. High minus lenses have thicker edges, because the inner lens surface's radius of curve is typically steeper than that of the outer surface, to create the correct lensing effect. If a lens with a steeper base curve is used for the front (outer) surface to accommodate a back-side progressive design, the lenses will be driven to even greater and undesired thicknesses, and will appear more bulbous than a normal minus prescription.
Difficulties and non-optimal tradeoffs may also arise when fitting back-surface progressive lenses into the frame. If a steeper base curve was used to accommodate the back-side add power, then one may wish to position the edge of the lens farther back in the frame so it is not so protuberant. However, this may cause contact of the inner surface or the edges of the inner surface with the wearer's face. In addition, depending on the frame shape or the wrap angle, it may be difficult to fit the lens securely or aesthetically. If thicker edges resulted from the back-side progressive design, it may be harder to make the glasses look appealing and balanced. Thus, there are several different concerns that may need to be addressed when employing a back-surface progressive design.
Sharing the power between the two surfaces can obviously offer more options for distributions of thickness and power profiles, but significantly increases the complexity of design, and thus may require higher costs, time and resources to prepare the final lens. In addition, depending on how the different portions of the prescription power are distributed, one may still have problems such as increased thickness on the inner surface (leading to contact with the face or eyelashes of the wearer), excess curvature on the front surface, and increased weight of the lenses.
It is apparent that extensive innovation continues in this field and new options often finds practical industrial applications quickly. The present invention provides a different option for ophthalmic lenses and their design, which is particularly suitable for lenses requiring different powers in different areas of the lens. This is accomplished by creating an innovative surface that is then combined with surfacing of the second surface to meet the optical requirements of an individual's prescription. The combination lens may also take into account other factors of cosmetic or practical design. This invention can advantageously use digital lens surfacing capabilities that have now reached a level of maturity such that they are accessible and practical for much of the lens industry.